I. Technical Field
The present invention relates to a flow sheet that is installed in the headbox of a paper machine to rectify the flow of paper stock within the headbox.
II. Description of the Related Art
When manufacturing paper with a paper machine, to improve the quality of paper to be manufactured, it is necessary to make the quantity of paper stock that is supplied from the headbox to the wire uniform. However, if the paper stock forms considerable turbulence within the headbox, the quantity of paper stock supplied from the headbox cannot be made uniform. Hence, to make the magnitude of a vortex flow of paper stock within the head box smaller, a rectifying sheet called a flow sheet is installed in the headbox.
This flow sheet, as disclosed in Japanese Patent Publication No. SHO 61-46597, is normally disposed within the headbox, with its upstream end fixed and its downstream end floated as a free end in the flow of paper stock. This rectifies the flow of paper stock within the headbox, whereby the quality of paper manufactured by the paper machine is improved. The fluid action and effects that are obtained by the use of the flow sheet within the headbox are described in detail in Japanese Patent Publication No. SHO 61-46597. Japanese Patent Publication No. SHO 61-46597 also discloses that the material of the flow sheet can use polycarbonate and carbon. Japanese Patent Publication No. SHO 63-50470 discloses a flow sheet in which rigidity can be designed in the flow direction (MD) and width (CD) direction by stacking layers of fibers.
Nowadays, flow sheets are manufactured by making prepregs in which carbon fibers are impregnated with resin, and stacking and bonding the prepregs. A prepreg is made into a thin sheet by disposing carbon fibers so that they intersect at right angles, or disposing them in parallel, and impregnating the disposed carbon fibers with resin. Next, a plurality of prepreg sheets are stacked within a mold form, the mold form is put in an autoclave, and within the autoclave the stack of prepreg sheets is heated under high pressure. The heated resin has a fluidity and fills gap between the prepregs. The unnecessary resin is removed from the mold, and the prepregs are bonded and hardened, whereby a flow sheet is molded. Note that before heating, air between the prepregs is removed by suction so that air bubbles do not remain between them.
To prevent fibers and fillers contained in a solution of stock, adhesive matter such as pitch, and adhesive matter due to the propagation of mold from adhering to the surface of a flow sheet, the flow sheet surface is required to be extremely smooth.
Polycarbonate flow sheets for the headbox spread in the 1970s and are still being widely used. Polycarbonate sheets, having a thickness of 1 to 3 mm and not joined in the longitudinal direction, are manufactured by resin makers and are relatively inexpensive and available. The representative tensile strength of the material is about 63 MPa. The arithmetical mean roughness Ra of the material is 0.1 μm or less, so it has excellent smoothness.
The thickness of the extreme end portion of a flow sheet is polished as thin as possible to reduce the eddies of wake flow that occurs on the extreme end portion. Considering strength, the extreme end portion is formed into a tapering shape so that the extreme end is 0.5 mm. In the case of a 3-mm polycarbonate sheet, it is tapered in the range of about 75 to 150 mm upstream from the extreme end.
The upstream end of the flow sheet is fitted in and bonded to a notch formed in a polycarbonate rod. By inserting the rod of the upstream end of the flow sheet into a groove provided in the interior of the headbox, the flow sheet is retained in flow.
Polycarbonate is high in corrosion resistance, but a machined surface is reduced in chemical resistance. Because there are cases where polycarbonate is degraded and cracked even by caustic washing that is performed at a concentration of about 1.5%, it is necessary to remove the flow sheet from the headbox.
To realize the hydraulic functions of the headbox, a technique of positively controlling flow by thickening the shape of a flow sheet was developed in the 1980s. Because of this, flow sheets of vinyl chloride were developed and put to practical use. Because the maximum length of a sheet of vinyl chloride is industrially 2.4 m, vinyl chloride sheets need to be welded and joined together in the longitudinal direction. A vinyl chloride flow sheet, as with a polycarbonate flow sheet, is tapered so that its extreme end is about 0.5 mm. Because vinyl chloride is high in causticity resistance, vinyl chloride flow sheets are not degraded by caustic washing. The smoothness of a machined surface of a vinyl chloride sheet becomes 0.2 to 0.4 μm in terms of arithmetical mean roughness Ra if it is polished. The representative tensile strength of a vinyl chloride sheet is about 55 MPa.
Carbon graphite flow sheets with a tensile strength of 300 to 700 MPa are available and have about five to ten times the strength of polycarbonate or vinyl chloride. Since carbon graphite flow sheets have such a strength characteristic, these flow sheets were developed in the mid-1980s and used partially.
In conventional carbon graphite flow sheets, before stacking, heating, and joining a plurality of sheet prepregs, vacuum suction is performed so that air bubbles do not remain between the prepregs.
In addition, to improve smoothness, carbon sheets are manufactured and then painted, whereby the arithmetical means roughness Ra can be increased up to 0.1 to 0.2 μm.